let me first start off by sayin that i'm 19. ok so here goes.
you can fit 5" mid-bass speakers in the "stock" mounting locations
provided that you trim a little of the door panel. "the stock location is
about 5" to the front of the van of the window crank and 4" down. you can
mount 1" tweeters where the a-pillar meets the roof.(above your head)
that should take care of the front 2 channels,

for the rear speakers you could do like the american conversion van's do
it.. a 6x9 in the sliding door, or you could mount 2 6x9's (in 6x9 truck
boxes) in the back of the van.. and have them be moveable.

just a suggestion..... oh yeah, and GET AN AMP :-)  not for the loudness,
but for the clarity. more power means clearer sound(with high quality
speakers)  also, a 8" woofer under the back seat is not a bad idea because
then you have all frquency ranges covered and you will get excellent sound
response.(most sub amps now come with a remote volume control as an
option so you can turn the sub down when you don't want as much bass)

good luck

Sudhir H. Desai          1984 VW Vanagon 3.4 liter Chevy V-6
desa4622@kettering.edu   sudhir_desai@geocities.com
ICQ# 17014903 & 8255956  http://www.geocities.com/MotorCity/Speedway/1889